Complications in the design of a motor vehicle drivetrain and its control sometimes result in various performance related compromises. Fortunately however, increased sophistication in engine controls (primarily spark timing and fuel) have enabled the performance compromises to be applied strategically. Spark timing, for example, is normally calibrated to maximize engine output torque, but is variably retarded from the maximum torque setting (referred to herein as the MBT setting) to avoid preignition and knock with fuels of the recommended octane rating. Spark retard may also be employed to meet drivetrain-related torque constraints.
In implementing controls of the above type, the goal is to avoid operating conditions which might damage the drivetrain or degrade the drivability of the vehicle, while maximizing performance and/or economy. However, the control tends to become unduly complicated and changes made to one part of the control often adversely affect another part of the control. For example, if an engine-related change permits the calibration engineer to advance the spark timing under certain operating conditions, the increased engine output torque may degrade the transmission shift quality or exceed a drivetrain torque limit. The opposite situation may also occur.